1、主将从现（主句为一般将来时,从句为一般现在时）；

eg:If it rains(从句) ,I will stay at home（主句）.

=I will stay at home if it rains.

2、主句含有情态动词 （must、may、can etc.）从句为一般现在时；

eg:If you drive too fast ,you may have an accident.

=You may have an accident if you drive too fast

3、主句为祈使句,从句要用一般现在时态；

eg:Dont play computer games if you dont finish your homework.